Most cancer deaths are caused by metastasis, a process whereby primary tumor cells spread to non-adjacent organs mainly by penetrating the walls of blood vessels and circulating through the bloodstream. Patients would have a much greater opportunity for long-term survival if these circulating tumor cells (CTCs) could be sensitively and specifically detected to guide disease management. However, CTCs are too rare for easy detection and quantification. Photoacoustic (PA) imaging following magnetic capture of circulating tumor cells has been proposed to address this problem, but the method is limited in contrast specificity due to strong PA signals from blood. Magnetomotive photoacoustic imaging (mmPA), a new molecular imaging modality developed in our group, introduced dynamic manipulation into traditional PA imaging. Similar to conventional PA, mmPA retains the high resolution and penetration of ultrasound (US), and can measure optical absorption in tissue. Unlike conventional PA, magnetomotive manipulation with simultaneous US/PA imaging of agents incorporating magnetic nanoparticles (MNPs) enables direct visualization of the signal generating object and can dramatically reduce background signals from strong optical absorbers such as blood. We hypothesize that biologically targeted, coupled magnetic nanoparticles can be used to identify, accumulate, and manipulate CTCs circulating in the vasculature using a combination of magnetic trapping and mmPA imaging. If successful, this technique can lead to a non-invasive system to accumulate CTCs, enabling highly sensitive CTC detection with a simple system appropriate for ultimate clinical translation. To test this hypothesis, a research plan with five specific aims has been developed. The first is to demonstrate that coupled MNPs targeted to mimics of circulating rare cells can be identified, accumulated, and manipulated in a vascular phantom using a combination of magnetic trapping and mmPA imaging. In the second aim, we will develop an effective magnetic trapping approach that can be easily integrated with a real-time US/PA imaging system appropriate for potential clinical applications in the peripheral vasculature. The third aim, in which a highly magnetic and NIR-absorbing coupled nanoprobe will be synthesized and characterized, is focused on developing the appropriate contrast agent for this application. Before performing in vivo tests, the fourth aim will demonstrate trapping and manipulation of targeted cells in circulation using an in vitro model of flow in a peripheral vessel. Finally, the overall approach will be validated i vivo by demonstrating trapping and manipulation of targeted cells in circulation using a murine model of metastatic cell trafficking in the vasculature. The overall goal of the proposed research plan is to help provide the background required to construct a prototype integrated system and to design studies helping translate mmPA technology into the clinic. This is a necessary first step in developing a robust system for metastatic disease management.